Nick Coghlan <[email protected]> added the comment:
The benefit offered by the parent local scoping was that it made assignment
expressions usable as a straightforward way to implement comprehension-based
accumulators where you actually do want access to the final value after the
comprehension completes (for example, pulling the example or counter-example
out of a call to any() or all()).
The downside is that you need an explicit "del j" after the comprehension to
ensure prompt cleanup in those cases where you *don't* need the temporary
variable after the comprehension has finished running:
>>> [(j:=i*i)+1/j for i in range(1, 3)]; del j # Clean up temp
However, that's still going to be clearer to most readers than writing:
[j+1/j for i in range(1, 3) for j in [i*i]]
So even with the parent local scoping semantics, PEP 572 is still enough to
make Yury's comment above still hold (i.e. the use case is too obscure to
justify the extra code needed to optimise it)
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<https://bugs.python.org/issue32856>
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